The present invention relates to improvements in self-attaching fasteners which significantly improves the strength of the installation of the fastener in a panel, such as the relatively thin steel panels used by the automotive and appliance industries. Although the improved fastening elements of this invention are described in regard to improved nut-type female fasteners, the improvements may also be applied to male stud-type fasteners as disclosed, for example, in U.S. Pat. No. 4,555,838.
The automotive industry utilizes self-piercing female fasteners which may also be simultaneously clinched to secure the nut in the pierce panel opening as described, for example, in U.S. Pat. Nos. 3,299,500 and 3,314,138 assigned to the assignee of the present application. The prior art also includes a number of self-riveting nuts, including nuts having an annular skirt or barrel portion, wherein the free end of the barrel is deformed radially outwardly in a die member to form a mechanical interlock with the panel as disclosed, for example, in U.S. Pat. Nos. 4,398,239 and 4,018,257. The self-riveting nuts disclosed in such patents are, however, secured to a panel in a pre-pierced panel opening, requiring two separate operations. The method of installing the nut fastener disclosed in these patents also require very precise centering of the nut relative to the pre-pierced panel opening, wherein a spring-biased pin is received through the panel opening and the nut is centered on the pin, prior to installation. U.S. Pat. No. 3,926,236, also assigned to the assignee of the present application, discloses a method of attaching a nut wherein the panel is pierced by a punch which extends through the nut bore to pierce and secure the nut in a continuous operation. However, the fastener is not a riveting-type fastener having a barrel portion extending through the pierced panel opening. Self-riveting fasteners have also been used by the container industry for attaching a closure flange or tag ring for reception of a threaded plug as disclosed, for example, in U.S. Pat. No. 3,800,401.
This application and the above-identified related patents and applications disclose methods and apparatus for permanently attaching male and female elements to a panel, such as nuts and bolts, in a continuous operation. The preferred methods of installation do not require pre-piercing of the panel, although the self-attaching fastening elements of this invention may be installed in a pre-pierced panel opening. Certain difficulties have, however, been experienced in installing female fastening elements in a panel, particularly where the barrel portion of the female element is used to pierce the panel opening. As will be understood, if the female fastening element is a nut-type fastener, the bore of the nut must remain clear of obstructions following installation for receipt of a stud or bolt. The method of installation and installation apparatus is preferably also suitable for mass production applications, such as used by the automotive and appliance industries.
Self-riveting fasteners of the type disclosed herein are particularly strong in resisting pull-out and cam-out. Pull is tested by threading a bolt in the threaded bore of the female fastener and pulling in a direction opposite to the direction of installation. The pull-out strength in many applications is greater than the tensile strength of the bolt. Cam-out is the strength of the installation to a cantilevered load. However, certain applications, such as automotive seat track and belt applications, require improvements in push-through strength and resistance to torque. Resistance to torque is generally provided by barbs or ribs on the body of the fastening element, which may result in stress cracks in the panel. The need therefore remains for a self-attaching fastener and method of installation which results in a very strong fastener and panel assembly, including improved torque resistance and push-through strength.